202 



ELEMENTARY CHEMICAL MICROSCOPY 



essential. Rulings are usually of two types, as shown in Figs. 

 124 and 125. Where type 124 is employed the entire field of 

 view may be counted but in type 125 it is better to call a " field " 

 that area comprised within the ruled square. This system is 

 preferable to that of employing a cell with ruled bottom referred 

 to below. An attachable mechancial stage will be found to be 



FIG. 124. 



FIG. 125. 

 Net Ruled Eyepiece Micrometers. 



FIG. 126. 



a great help in avoiding the making of counts in the same area 

 more than once. 



Although the method just described appears at first sight to 

 be crude and unreliable it has been found after a number of 

 years' trial in the hands of a large number of students to yield 

 excellent results. 



In the case of starch mixtures, where the foreign component 

 is present in the proportion of 3 to 7 per cent the results found 

 are very close to the actual per cent, but when 7 per cent is 

 reached, the beginner has trouble in obtaining reliable counts, 

 and above 10 per cent the method requires great manipulative 

 skill. 



It must, however, be borne in mind that a method of this 

 sort even at its best gives merely a close approximation to the 

 true value. 



The chief difficulties which will be encountered are those of 

 removing equal amounts in every case upon the end of a tiny 

 spatula; of obtaining a uniform distribution of the material 

 throughout the drop; and of lowering the cover- glass upon the 



